Several devices are known that ionize a gaseous sample and analyze the product ions for the molecular makeup of the sample. The devices fall into two categories: those that operate under vacuum and those that operate under pressure conditions. The devices that operate under vacuum are know as mass spectrometers and separate ions according to charge-to-mass ratios using a combination of electromagnetic fields. The devices that operate under pressure are known as ion mobility spectrometers and separate ions according to mobilities through a drift gas in a constant electric field. Generally, mass spectrometers require a vacuum better than 10.sup.-3 mm Hg to eliminate the adverse effects of collisions between ions and neutral gas molecules. This is unlike ion mobility spectrometry where pressures greater than 10.sup.-3 mm Hg are needed to assure that collisions between the ions and neutral gas molecules firmly establish mobility values. Due to the absence of collisions in mass spectrometry, the ions can gain considerable energy as they respond to the imposed electromagnetic fields. In ion mobility spectrometry, the energy gained by the ions is rapidly dissipated by collisions between the ions and neutral gas molecules. One consequence of this difference in ion energy between mass spectrometry and ion mobility spectrometry is that the energetic ions of mass spectrometry do not follow electric field lines, while the thermal ions of ion mobility spectrometry do. Because of this difference, attempts to separate ions using one technique in the pressure regime of the other is generally unsuccessful. On the other hand, there are enough similarities between mass spectrometry and ion mobility spectrometry to encourage exploitation of common features.
The technique of ion mobility spectrometry (IMS) was first disclosed in U.S. Pat. No. 3,699,333 which issued on Oct. 17, 1972 to M. J. Cohen, D. I. Carroll, R. F. Wemlund and W. D. Kilpatrick. It was originally conceived as a method to analyze and detect organic vapors in a gas mixture. FIG. 1 shows a simplified IMS detector cell. It contains two regions: a reaction (or reactor) region where the ions are ionized, and a drift (or drift tube) region where the ions are separated. The ionization and separation processes occur under a wide range of pressure to, conditions, but the preferred operating pressure in U.S. Pat. No. 3,699,333 was atmospheric pressure. In the reaction region, the sample is either ionized directly by using ultraviolet radiation from a photoionization source, electrospraying the ions as a mist into the ionizer, etc.; or indirectly by reacting with an intermediate set of reactant ions (designated by R.sup..+-. in FIG. 1). The indirect method of ionization is known as chemical ionization and the reactant ions are created by using a radioactive source (e.g., Ni.sup.63, Am.sup.241, tritium, etc.), a corona discharge source, a thermionic emitter of alkali ions, or another primary source of ions.
The nature of the reactant ions generated by the ionization source depends on the composition of the carrier gas used to transport sample into the reactor of the ion mobility spectrometer. This dependency can be used to selectively ionize a specific component in a sample matrix by adjusting the composition of the carrier gas. This is accomplished by doping the carrier gas with a low level of a chemical reagent, such as acetone, a chlorinated solvent, methyl salicylate, etc. The reactant ions then become a protonated di,acetones a chloride anion or a protonated monomer of methyl salicylate, etc. that react differently the ample.
While the reactant ions and product ions (designated by P.sup..+-. in FIG. 1) can be positively or negatively charged, the polarity of the ions that are extracted from the reactor and analyzed by the drift tube depends upon the directionality of the electric field applied to the drift tube. If the ionization source is biased positive relative to the ion collector, positive ions are extracted from the reactor and analyzed by the drift tube for mobility. If the ionization source is biased negative relative to the ion collector, negative ions are extracted from the reactor and analyzed by the drift tube for mobility. If no electric field is applied, the positive and negative ions recombine, and are otherwise lost for analysis by the drift tube.
A shutter grid positioned between the reactor and the drift tube provides a means whereby a localized concentration of ions is extracted from the reactor and introduced into the drift tube. Typically this shutter grid consists of a planar array of parallel wires with neighboring wires electrically independent. When the two sets of electrically independent wires are at the same potential, the ions pass freely through the grid and enter the drift tube. When the two sets of neighboring wires are at different potentials, the ions are captured by the grid and are denied entry into the drift tube. Ion injection into the drift tube is accomplished by momentarily removing the blocking potential from the shutter grid. Once inside the drift tube and exposed to the drift field applied to the drift tube, the ions migrate toward an ion collector (or Faraday plate) located at the other end of the drift tube. When the ions arrive at the collector, their drift time is recorded and correlated with the composition of the original sample delivered to the reactor.
The IMS technique, as described above, has several limitations. These include:
1. The basic limits of detection are restricted to about ten picograms or ten parts per trillion due to build up of space charge in the reactor. There is no capability of concentrating and storing ions. PA1 2. Ion mobilities are sensitive to the composition of the drift gas, and decrease as the ion clusters with water vapor or other polar compounds. Ions attached to contaminant gases have different mobilities, making it difficult to identify the ions. PA1 3. Miniature IMS sensors are plagued by low total ion currents (the ion current collected by the ion collector when the shutter grid is biased open continuously) that limit the dynamic range of the device.
U.S. Pat. No. 5,200,614 which issued on Apr. 6, 1993 to A. Jenkins and W. J. McGann describes an "ion trap mobility spectrometer" that attempts to remove one of the above limitations and improves the limits of detection of IMS for electrophilic compounds (e.g., nitro-compounds used as explosives). A schematic representation of their device is shown in FIG. 2. The two halves of the shutter grid are separated to create a field-free ion storage region within the device. When the two grids (E1 and E2 in FIG. 2) are at the same potential, the ions entering the ion storage region from the reactor become "trapped" (i.e., lie motionless). By momentarily applying a high potential between grids E1 and E2 (V.sub.3 in FIG. 2), the "trapped" ions are injected into the drift tube. The ion storage region, therefore, behaves like a pulsed reactor for the IMS. When compared to the reactor of a conventional IMS, this pulsed reactor has the advantage that it increases the reaction time for ionization; and in the case where electron capture processes are important, thermalizes the reacting electrons. On the other hand, the disadvantages are that space charge can more easily build up in the ion storage region. Like conventional IMS, the drift times for the ions in the affected ion trap mobility spetrometer are by the composition of the carrier gas flowing through the drift region, and the total ion current decreases when attempts are made to miniaturize the cell.
Disclosed in Russian Inventor's Certificate No. 9666583 is another method for separating ions according to mobility. The parallel plate ion separator for this invention is shown in FIG. 3 where the ion flow (induced by a drift gas) is from left to right. A transverse asymmetric AC field is applied across the electrodes of the separator to excite a perpendicular micromotion in the ions as they move in the average direction of the flowing drift gas. The amplitude of the micromotion is proportional to the electric field strength through a mobility coefficient K. The relationship v.sub.d =KE is a vector relationship between the drift velocity v.sub.d of the ion and the electric field E. Unlike conventional IMS where ion separation is accomplished using relatively low electric field strengths (e.g., 150-250 volts/cm), higher field strengths are required to successfully separate the ions in the Russian invention. E. A. Mason and E. W. McDaniel in their book entitled "Transport Properties of Ions in Gases" (Wiley, New York, 1988) teach that the mobility coefficient K is a function of the electric field E. An approximate expression for the functional dependence is K(E)=K.sub.0 +K.sub.2 E.sup.2 +K.sub.4 E.sup.4 + . . . , where the K.sub.1 's are coefficients dependent on the ion species under consideration. Therefore when an ion is exposed to an asymmetric AC potential that is oscillating between adequately high and low values, the ion experiences different mobilities when traveling in one direction compared to the other. This causes; the ions to move more in one direction than another, and be neutralized when they collide with the electrodes of the Russian invention. By adding a DC component to the asymmetric potential, the path taken by the ions can be altered; and depending on the combination of the DC and asymmetric AC potentials applied, certain ions can be directed towards an ion collector. Since the difference in mobilities created by the asymmetric field is dependent on the type of ion being analyzed, ion separation is possible.
In a paper published in the International Journal of Mass Spectrometry and Ion Processes; volume 128 (1993), pp. 143-148; I. A. Buryakov, E. V. Krylov, E. G. Nazarov and U. Kh. Rasulev further describe the method of Certificate No. 9666583. They state that the ion separation is performed in a dense gas (e.g., air at 760 mm Hg) using a 2 megahertz RF potential. The waveform for the RF potential is rectangular with a period of T=t.sub.1 +t.sub.2 (t.sub.1 &lt;&lt;t.sub.2); the absolute value for the positive semi-period, t.sub.1 (E.sub.max), being much less than the absolute value for the negative semi-period, t.sub.2 (E.sub.min), and the integrated areas for the waveform above and below zero being equal. An ion spectrum (sometimes called an ionogram) is obtained by superimposing the asymmetric potential on top of a DC potential and scanning the DC potential. As the DC potential is scanned, ions with different mobilities sequentially pass through the device. Buryakov, et al. showed that amines in a gas mixture can be selectively detected within 10 seconds. They further stated that because of its small size, the parallel plate ion separator can be incorporated into a portable gas analyzer.
The ion separator of FIG. 3, however, has several disadvantages. The linear velocity of the drift gas must be kept constant across the diameter of the tube and also, preferably, along its length. Diffusers are required to establish laminar flow conditions. In addition, the device has a slow response because the velocity of the ions along the longitudinal direction of the drift tube is controlled by the relatively slow moving drift gas.
In U.S. Pat. No. 5,420,424 which issued on May 30, 1995, B. L. Carnahan and A. S. Tarassov disclosed a modified version of the parallel plate ion separator which they called a "transverse field ion mobility spectrometer" (later concatenated to "field ion spectrometer (FIS)") This device is shown in FIG. 4 which is a cross-sectional view of cylindrical geometry. The cylindrical capacitor provides a more uniform field and a greater cross-sectional area for transmission of ions. Instead of using the rectangular waveform of Buryakov, et al., they used an oscillating potential is superimposed upon its second harmonic to generate the asymmetric field; i.e., V(t)=V.sub.0 +V.sub.1 [(1-.beta.) cos .omega.t+.beta. cos 2.omega.t], where V.sub.0 and V.sub.1 are constants and 0.1&lt;.beta.&lt;0.7. The field ion spectrometer has been used to collect spectra on various organo phosphorus and aromatic compounds with a total analysis time of 0.1 to 1.3 seconds. However, the device continues to suffer from the deficiencies noted above for the parallel plate ion separator of FIG. 3.
Despite the fact that they work only under vacuum conditions, certain mass spectrometers Be are related to the devices of FIGS. 1-4. An important mass spectrometer for this purpose is the quadrupole mass filter first disclosed by W. Paul, et al. in U.S. Pat. Nos. 2,939,952 and 2,950,389 which issued on Jun. 7, 1960 and Aug. 30, 1960, respectively. Such a mass filter is illustrated in FIG. 5. The vacuum allows the focusing lenses to accelerate the ions and direct them onto the entrance aperture of four quadrupole rods. Since the function of the quadrupole rods is to separate ions, they are sometimes collectively referred to as a quadrupole ion filter. Again due to the vacuum, the ions maintain their linear velocity as they pass through the quadrupole filter; and as they interact with an oscillating electromagnetic field applied across the rods, they oscillate perpendicular to their original direction of motion. The oscillating field is created by a symmetric RF and DC potential applied across neighboring rods. The magnitude of the ion oscillation is dependent on the mass-to-charge ratio of the ions; and because the oscillations are so great, most of the ions hit the quadrupole rods. Certain of the ions, however, do not hit the rods and survive until they reach the opposite end of the filter. A detector, or electron multiplier, registers the arrival of the surviving ions.
The principle of operation for the quadrupole mass filter relies upon the electric field applying a restoring force to the ion so that it oscillates about some preferred position within the rods. To effectively perform this function, the electric field must satisfy certain spatial distribution requirements. In particular, the electric field must be quadrupolar. Such a field is created by carefully sculpting the internal surfaces of the quadrupole rods. Theoretically, the internal surfaces should define complementary hyperbolas. However, due to difficulties in machining hyperbolas, the hyperbolic rods are often replaced with round rods, (as shown in FIG. 5) that are carefully placed relative to each other to create the desired hyperbolic field.
An ion separator related to the quadrupole mass filter is the monopole mass filter described by U. von Zahn in a paper published in the Review of Scientific Instruments, volume 34 (1963), pp. 1-4. Such a filter is shown in FIG. 6. The monopole mass filter is a rod and an angle electrode located relative to each other so that a quarter-section of the quadrupole mass filter is approximated. Ions are separated by applying a combination of RF and DC potentials across the two electrodes. Because the angle electrode occupies the path that t:he ions would normally travel in a quadrupole mass filter, the ions are injected with a transverse, as well as a longitudinal, velocity component into the monopole mass filter. After injection, the ions describe an arc; first moving toward the rod, and then away from the rod and toward the angle electrode. Like the quadrupole mass filter, the combination of the RF and DC potentials determines which ions pass through the electrode structure for detection by an electron multiplier.
In addition to the quadrupole mass filter, W. Paul and H. Steinwedel al so disclosed a three-dimensional analogue of the quadrupole mass filter in U.S. Pat. No. 2,939,952. This variation eventually became know as the "ion trap mass spectrometer (ITMS)" further disclosed by G. C. Stafford, P. E. Kelley and D. R. Stephens in U.S. Pat. No. 4,540,884, dated Sep. 10, 1985, and shown in FIG. 7. Being a cylindrical analogue of the linear quadrupole filter, the electrode structure for the ITMS consists of a ring-electrode sandwiched between two end-caps with the internal surfaces defining revolutions of complementary hyperbolas. Although other shapes for the electrode structures have been studied, more attention has been given to fabricating ideal electrode shapes (albeit with known distortions) for the ITMS than for the linear quadrupole.
When a symmetric RF and DC (optional) potential is applied across the ring and end-cap electrodes of the ITMS, a trapping field develops within its volume. The trapping field is characterized by a potential well (more specifically, a rotating saddle point) that causes the ions to migrate towards and oscillate around the center of the trap. This trapping field is considerably different from the trapping field described earlier for the field free region of the ion trap mobility spectrometer of FIG. 2. Unlike the ion trap mobility spectrometer, the restoring force invoked by the rotating saddle point causes the ions to be trapped for longer periods of time near the center of the trap. In fact, the times are so long that the ITMS is sometimes referred to as an "ion storage trap", or "ion store" for short. On the other hand, the ions can be ejected at will by changing the combination of the RF and DC potentials applied to the electrodes. This combination of storing and then releasing ions allows ion concentrations (see U.S. Pat. No. 4,650,999, dated Mar. 17, 1987 for handling space charge effects) to be enriched before they are delivered to a detector. It also allows the ITMS to work as a mass spectrometer.
Since the original ion trap inventions, several investigators have disclosed that the trapping field does not have to be quadrupolar. For example, J. Franzen, et al. in U.S. Pat. Nos. 4,882,484; 4,975,577; 5,028,777; 5,170,054; 5,283,436; 5,331,157; 5,386,113 and 5,468,958 state that multipolar fields can be added to the quadrupolar field to create nonlinear resonances that improve mass resolution, scan speed and ion storage stability. In chapter 3 of a book entitled "Practical Aspects of Ion Trap Mass Spectrometry: Volume I" (CRC Press: Boca Raton, Fla., 1995), it is disclosed that the nonlinear resonances can be unintentionally introduced by imperfect machining of the electrodes, or intentionally introduced to improve the performance of the trap.
Examples of electrode structures that can be used to generate multipolar fields are shown in FIGS. 8A-8C. FIG. 8A is the conventional quadrupole structure, while FIGS. 8B and 8C are the hexapole and octapole structures, respectively. The analytical expressions for the electric field and potential created by each of the structures are also shown in FIG. 8. "z" represents the vertical longitudinal dimension and "r" represents the horizontal radial dimension in each case. E.sub.r and E.sub.z represent the electric field in the r and z directions respectively, and .phi. represents the potential. To create a nonlinear resonance, a weighted sum of two or more of these fields is necessary.
Other electrode structures that have been investigated for vacuum-operated ion trap mass spectrometers are shown in FIG. 9 along with the equipotential lines they generate. Each trap is a cross-sectional view of cylindrical geometry where the electrodes are indicated by cross-hatched areas. FIG. 9A is the conventional quadrupole structure and FIGS. 9B and 9C are two planar analogues. Brewer, DeVoe and Kallenbach have theoretically analyzed the planar traps in a paper published in the journal entitled Physical Review A, volume 46 (1992), pp. R6781-6784. In each case, the electrode(s) corresponding to the ring-electrode is/are labeled as 1, and the electrodes corresponding to the end-caps as 2. It is evident that the equipotential line profile is largely quadrupolar in each case. It is also evident that the planar analogues, have other multipolar contributions.
To date, none of the mass spectrometer devices of FIGS. 5-9 have been successfully used to separate ions at pressures greater than about 10.sup.-2 mm Hg. Any attempt to do so results in a loss of signal. The reason for the loss in signal is that the ions collide with, and are scattered by, the neutral gas molecules contained in the analyzer. The collision and scattering events prevent the ions from reaching the ion collector.